1. Field of the Invention
The present invention relates to a communication system performing cryptographic communication in which digital contents, such as music, images, videos, and games, having a decryption limitation are transferred using a common key which is shared by devices so that the decryption of the digital contents is forbidden when the updating of the decryption limitation is unauthorized. More particularly, the present invention relates to a copyright protection system, an encryption device, a decryption device, and a recording medium for protecting copyrights by associating update information on the decryption limitation with the common key.
2. Description of the Related Art
Recently, the development of digital information compression technologies and the explosive pervasion of communication infrastructures have realized that contents, such as music, images, videos, and games, are distributed in the form of digital information via communication lines to homes.
The digital information distributed via communication lines is in the form of data which is not stored in any medium. Therefore, the flexibility of distribution service forms is dramatically increased. Distribution services can not only provide digital contents but also limit the use of the contents (e.g., the limited number of uses and the limited period of use). A wide variety of distribution service forms are contemplated.
The establishment of distribution systems, in which the copyrights of digital contents and the profits of distributors are protected, requires solving how to prevent unauthorized actions, such as fraud possession by communication intercept, eavesdropping, pretending, or the like, and illegal duplications and falsifications of received data stored in a recording medium. Such a solution would be provided by copyright protection technologies, such as an encryption/authentication technique performing the identification of authentic systems, data scramble, and the like.
There are a variety of conventional copyright protection technologies. A typical technology is a challenge-response type mutual authentication system in which random numbers and response values are exchanged between a data encryption device and a data decryption device so that both devices are mutually authenticated, and data is transferred when the authentication is established.
The term “decryption limitation” as used herein refers to information on whether contents transferred from an encryption device to a decryption device are allowed to be used (e.g., reproduce to make a sound). For example, when contents are associated with the number of times the contents can be reproduced, such a number of times is a decryption limitation.
The term “updating of a decryption limitation” as used herein refers to a rule which is used in updating a decryption limitation. For example, for contents associated with the number of times the contents can be reproduced (e.g., N times), such a number of times is a decryption limitation transferred from an encryption device to a decryption device, and the updating of the decryption limitation means that the number of times is reduced by one.
The term “update information on a decryption limitation” as used herein refers to a decryption limitation which is updated. For example, for contents associated with the number of times the contents can be reproduced (e.g., N times), the number of times which is a decryption limitation transferred from an encryption device to a decryption device, is updated so that the update information on the decryption limitation is rewritten to “N−1 times”.
A typical cryptographic communication system in which digital contents having a decryption limitation are transferred using the above-described mutual authentication technique, will be described. An encryption device and a decryption device are mutually authenticated. Only when the authentication is established, the decryption limitation is encrypted and then transferred from the encryption device to the decryption device. The decryption device interprets the decryption limitation to determine whether the digital contents can be decrypted, and updates the decryption limitation. The update information on the updated decryption limitation is encrypted and transferred to the encryption device. Thereafter, the contents are encrypted and loaded into the decryption device which in turn decrypts the loaded contents.
A decryption limitation should be correctly updated. In other words, update information on a decryption limitation decrypted by a decryption device should be received by an authenticated encryption device. If a decryption limitation is not correctly updated, i.e., update information on a decryption limitation decrypted by a decryption device is received by a false encryption device pretending to be an authenticated encryption device, the decryption limitation is not updated by the authenticated encryption device and contents loaded from the authenticated encryption device are decrypted by the decryption device in an unauthorized manner. Therefore, a system is required in which, when update information on a decryption limitation decrypted by a decryption device is received by a false encryption device pretending to be an authenticated encryption device, the decryption device is forbidden to decrypt contents loaded from the authenticated encryption device.
In the above-described mutual authentication technique, a determination is made only as to whether communicating devices are authenticated. Whether a decryption limitation is currently updated is not determined. Therefore, an unauthorized action cannot be prevented. If update information on a decryption limitation decrypted by a decryption device is received by a false encryption device pretending to be an authenticated encryption device, the decryption limitation is not updated by the authenticated encryption device, and nevertheless contents loaded from the authenticated encryption device cannot be decrypted by the decryption device in an unauthorized manner.